Implantable medical devices for therapeutic stimulation of the heart are well known in the art. In U.S. Pat. No. 4,253,466 issued to Hartlaub et al., for example, a programmable demand pacemaker is disclosed. The demand pacemaker delivers electrical energy, typically ranging in magnitude between about 5 and about 25 micro Joules, to the heart to initiate the depolarization of cardiac tissue. This stimulating regime is used to treat heart block by providing electrical stimulation in the absence of naturally occurring spontaneous cardiac depolarizations.
Another form of implantable medical device for therapeutic stimulation of the heart is an automatic implantable defibrillator (AID), such as those described in U.S. Pat. No. Re. 27,757 to Mirowski et al. and U.S. Pat. No. 4,030,509 to Heilman et al. Those AID devices deliver energy (about 40 Joules) to the heart to interrupt ventricular fibrillation of the heart. In operation, an AID device detects the ventricular fibrillation and delivers a nonsynchronous high-voltage pulse to the heart through widely spaced electrodes located outside of the heart, thus mimicking transthoracic defibrillation. The technique of Heilman et al. requires both a limited thoracotomy to implant an electrode near the apex of the heart and a pervenous electrode system located in the superior vena cava of the heart.
Another example of a prior art implantable cardioverter includes the pacemaker/cardioverter/defibrillator (PCD) disclosed in U.S. Pat. No. 4,375,817 to Engle et al. This device detects the onset of tachyarrhythmia and includes means to monitor or detect the progression of the tachyarrhythmia so that progressively greater energy levels may be applied to the heart to interrupt a ventricular tachycardia or fibrillation.
Another device is an external synchronized cardioverter, such as that described in "Clinical Application of Cardioversion" in Cardiovascular Clinics, 1970, Vol. 2, pp. 239-260 by Douglas P. Zipes. This type of external device provides cardioversion shocks synchronized with ventricular depolarization to ensure that the cardioverting energy is not delivered during the vulnerable T-wave portion of the cardiac cycle.
Another example of a prior art implantable cardioverter includes the device disclosed in U.S. Pat. No. 4,384,585 to Douglas P. Zipes. This device includes circuitry to detect the intrinsic depolarizations of cardiac tissue and pulse generator circuitry to deliver moderate energy level stimuli (in the range of about 0.1 to about 10 Joules) to the heart synchronously with the detected cardiac activity.
The functional objective of such a stimulating regimen is to depolarize areas of the myocardium involved in the genesis and maintenance of re-entrant or automatic tachyarrhythmias at lower energy levels with greater safety than was possible with nonsynchronous cardioversion. Nonsynchronous cardioversion always incurs the risk of precipitating ventricular fibrillation and sudden death. Synchronous cardioversion delivers the shock at a time when the bulk of cardiac tissue is already depolarized and is in a refractory state. Other examples of automatic implantable synchronous cardioverters include those of Charms in U.S. Pat. No. 3,738,370.
It is expected that the increased safety deriving from use of lower energy levels and their attendant reduced trauma to the myocardium, as well as the smaller size of implantable medical devices, will expand indications for use beyond the existing patient base of automatic implantable defibrillators. Since many episodes of ventricular fibrillation are preceded by ventricular (and in some cases, supraventricular) tachycardias, prompt termination of the tachycardia may prevent ventricular fibrillation.
Consequently, current devices for the treatment of tachyarrhythmias include the possibility of programming staged therapies of antitachycardia pacing regimens, along with cardioversion energy and defibrillation energy shock regimens in order to terminate the arrhythmia with the most energy-efficient and least traumatic to therapies, when possible. In addition, some current implantable tachycardia devices are capable of delivering single or dual chamber bradycardia pacing therapies, as of which are described, for example, in U.S. Pat. No. 4,800,833 to Winstrom, U.S. Pat. No. 4,830,006 to Haluska et al., and U.S. Pat. Application Serial No. 07/612,758 to Keimel for "Apparatus for Delivering Single and Multiple Cardioversion and Defibrillation Pulses" filed Nov. 14, 1990, and incorporated herein by reference in its entirety. Furthermore, and as described in the foregoing '833 and '006 patents and the '758 application, considerable study has been undertaken to devise the most efficient electrode systems and shock therapies.
Initially, implantable cardioverters and defibrillators were envisioned as operating with a single pair of electrodes applied on or in the heart. Examples of such systems are disclosed in the aforementioned '757 and '509 patents, wherein shocks are delivered between an electrode is placed in or on the right ventricle and a second electrode placed outside the right ventricle. Studies have indicated that two electrode defibrillation systems often require undesirably high energy levels to effect defibrillation.
In an effort to reduce the amount of energy required to effect defibrillation, numerous suggestions have been made with regard to multiple electrode systems.
Some of those suggestions are set forth in U.S. Pat. No. 4,291,699 to Geddes et al., U.S. Pat. No. 4,708,145 to Tacker et al., U.S. Pat. No. 4,727,877 to Kallock, and U.S. Pat. No. 4,932,407 issued to Williams where sequential pulse multiple electrode systems are described. Sequential pulse systems operate based on the assumption that sequential defibrillation pulses delivered between differing electrode pairs have an additive effect such that the overall energy requirements to achieve defibrillation are less than the energy levels required to accomplish defibrillation using a single pair of electrodes.
An alternative approach to multiple electrode sequential pulse defibrillation is disclosed in U.S. Pat. No. 4,641,656 to Smits and also in the above-cited '407 patent. This defibrillation method may conveniently be referred to as a multiple electrode simultaneous pulse defibrillation method, and involves the simultaneous delivery of defibrillation pulses between two different pairs of electrodes. For example, one electrode pair may include a right ventricular electrode and a coronary sinus electrode, and a second electrode pair may include a right ventricular electrode and a subcutaneous patch electrode, with the right ventricular electrode serving as a common electrode to both electrode pairs. An alternative multiple electrode, single path, biphasic pulse system is disclosed in U.S. Pat. No. 4,953,551 to Mehra et al., which employs right ventricular, superior vena cava and subcutaneous patch electrodes.
In the above-cited prior art simultaneous pulse multiple electrode systems, delivery of simultaneous defibrillation pulses is accomplished by simply coupling two electrodes together. For example, in the above-cited '551 patent, the superior vena cava and subcutaneous patch electrodes are electrically coupled together and a pulse is delivered between those two electrodes and the right ventricular electrode. Similarly, in the above-cited '407 patent, the subcutaneous patch and coronary sinus electrodes are electrically coupled together, and a pulse is delivered between these two electrodes and a right ventricular electrode. See also U.S. Pat. Nos. 5,411,539; 5,620,477; 5,6589,321; 5,545,189 and 5,578,062, where active can electrodes are discussed.
The aforementioned '758 application discloses a pulse generator for use in conjunction with an implantable cardioverter/defibrillator which is capable of providing all three of the defibrillation pulse methods described above, with a minimum of control and switching circuitry. The output stage is provided with two separate output capacitors which are sequentially discharged during sequential pulse defibrillation and simultaneously discharged during single or simultaneous pulse defibrillation. The complexity of those stimulation therapy regimens require rapid and efficient charging of high voltage output capacitors from low voltage battery power sources incorporated within the implantable medical device.
Typically, the electrical energy required to power an implantable cardiac pacemaker is supplied by a low voltage, low current drain, long-lived power source such as a lithium iodine pacemaker battery of the type manufactured by Wilson Greatbatch, Ltd. or Medtronic, Inc. While the energy density of such power sources is typically relatively high, they are generally not capable of being rapidly and repeatedly discharged at high current drains in the manner required to directly cardiovert the heart with cardioversion energies in the range of 0.1 to 10 Joules. Moreover, the nominal voltage at which such batteries operate is generally too low for cardioversion applications. Higher energy density battery systems are known which can be more rapidly or more often discharged, such as lithium thionyl chloride power sources. Neither of the foregoing battery types, however, may have the capacity or the voltage required to provide an impulse of the required magnitude on a repeatable basis to the heart following the onset of tachyarrhythmia.
Generally speaking, it is necessary to employ a DC-DC converter to convert electrical energy from a low voltage, low current power supply to a high voltage energy level stored in a high energy storage capacitor. A typical form of DC-DC converter is commonly referred to as a "flyback" converter which employs a transformer having a primary winding in series with the primary power supply and a secondary winding in series with the high energy capacitor. An interrupting circuit or switch is placed in series with the primary coil and battery. Charging of the high energy capacitor is accomplished by inducing a voltage in the primary winding of the transformer creating a magnetic field in the secondary winding. When the current in the primary winding is interrupted, the collapsing field develops a current in the secondary winding which is applied to the high energy capacitor to charge it. The repeated interruption of the supply current charges the high energy capacitor to a desired level over time.
In U.S. Pat. No. 4,548,209 to Wielders et al. and in the above-referenced '883 patent, charging circuits are disclosed which employ flyback oscillator voltage converters which step up the power source voltage and apply charging current to output capacitors until the capacitor voltage reaches a programmed shock energy level.
In charging circuit 34 of FIG. 4 in the '209 patent, two series-connected lithium thionyl chloride batteries 50 and 52 are connected to primary coil 54 of transformer 56 and to power FET transistor switch 60. Secondary coil 58 is connected through diode 62 to cardioversion energy storage capacitor 64. In this circuit, the flyback converter works generally as follows: When switch 60 is closed, current I.sub.p passing through primary winding 54 increases linearly as a function of the formula V.sub.p =L.sub.p dl/dt. When FET 60 is opened, the flux in the core of transformer 56 cannot change instantaneously, and so complimentary current is (which is proportional to the number of windings in primary and secondary coils 54 and 58, respectively) starts to flow in secondary winding 58 according to the formula I.sub.s =(N.sub.p /N.sub.s)I.sub.p.
Simultaneously, voltage in the secondary winding is developed according to the function V.sub.s =L.sub.s dl.sub.s /dt, thereby causing charging of cardioversion energy storage capacitor 64 to a programmed voltage.
The Power FET 60 is switched "on" at a constant frequency of 32 KHz for a duration or duty cycle that varies as a function of the voltage of the output capacitor reflected back into the primary coil 54 circuit. The on-time of power FET 60 is governed by the time interval between the setting and resetting of flip-flop 70, which in turn is governed either by current I.sub.p flowing through primary winding 54 or as a function of a time limit circuit containing further circuitry to vary the time limit with battery impedance (represented schematically by resistor 53). In both cases, the on-time varies from a maximum to a minimum interval as the output circuit voltage increases to its maximum value.
The aforementioned '883 and '006 patents disclose a variable duty cycle flyback oscillator voltage converter, where the current in the primary coil circuit (in the case of the '883 patent) or the voltage across a secondary coil (in the case of the '006 patent) is monitored to control the duty cycle of the oscillator. In the '883 circuit the "on" time of the oscillator is constant and the "off" time varies as a function of the monitored current through the transformer.
In the '006 patent, a secondary coil is added to power a high voltage regulator circuit that provides V+ to a timer circuit and components of the high voltage oscillator. This high voltage power source allows the oscillator circuit to operate independently of the battery source voltage (which may deplete over time). The inclusion of a further secondary winding on an already relatively bulky transformer is disadvantageous from size and efficiency standpoints.
Energy, volume, thickness and mass are critical features in the design of implantable cardiac defibrillators (ICDs). One of the components important to optimization of those features is the high voltage capacitors used to store the energy required for defibrillation. Such capacitors typically deliver energy in the range of about 25 to 40 Joules, while ICDs typically have a volume of about 40 to about 60 cc, a thickness of about 13 mm to about 16 mm and a mass of approximately 100 grams.
It is desirable to reduce the volume, thickness and mass of such capacitors and devices without reducing deliverable energy. Doing so is beneficial to patient comfort and minimizes complications due to erosion of tissue around the device. Reductions in size of the capacitors may also allow for the balanced addition of volume to the battery, thereby increasing longevity of the device, or balanced addition of new components, thereby adding functionality to the device. It is also desirable to provide such devices at low cost while retaining the highest level of performance.
Most ICDs employ commercial photoflash capacitors similar to those described by Troup in "Implantable Cardioverters and Defibrillators," Current Problems in Cardiology, Volume XIV, Number 12, Dec. 1989, Year Book Medical Publishers, Chicago, and U.S. Pat. No. 4,254,775 for "Implantable Defibrillator and Package Therefor". The electrodes in such capacitors are typically spirally wound to form a coiled electrode assembly. Most commercial photoflash capacitors contain a core of separator paper intended to prevent brittle anode foils from fracturing during coiling. The anode, cathode and separator are typically wound around such a paper core. The core limits both the thinness and volume of the ICDs in which they are placed. The cylindrical shape of commercial photoflash capacitors also limits the volumetric packaging efficiency and thickness of an ICD made using same.
As noted above, electrodes and separators used in the assembly of photoflash capacitors are typically coiled, with a resulting cylindrical capacitor geometry. Anodes employed in photoflash capacitors typically comprise one or two layers of a high purity (99.99%), porous, highly etched, anodized aluminum foil. Cathode layers in such capacitors are formed of a nonporous, highly etched aluminum foil which may be somewhat less pure (99.7%) respecting aluminum content than the anode layers. The thickness of such foils is on the order of 100 micrometers and 20 micrometers for anode foils and cathode foils, respectively. The capacitance of the cathode is balanced respecting that of the anode to ensure reliable performance over the life of the device. Separating the anode and cathode is a separator material that typically comprises two layers of Kraft paper.
Prior art electrolytic capacitors generally include a laminate comprising an etched aluminum foil anode, an aluminum foil of film cathode and a Kraft paper or fabric gauze spacer impregnated with a solvent based liquid electrolyte interposed therebetween. A layer of oxide is formed on the aluminum anode, preferably during passage of electrical current through the anode. The oxide layer functions as a dielectric layer. The entire laminate is rolled up into the form of a substantially cylindrical body and encased, with the aid of suitable insulation, in an aluminum tube or can subsequently sealed with a rubber material.
The energy of the capacitor is stored in the electromagnetic field generated by opposing electrical charges separated by an aluminum oxide layer disposed on the surface of the anode. The energy so stored is proportional to the surface area of the aluminum anode. Thus, to minimize the overall volume of the capacitor one must maximize anode surface area per unit volume without increasing the capacitor's overall (i.e., external) dimensions. Separator material, anode and cathode terminals, internal packaging and alignment features and cathode material further increase the thickness and volume of a capacitor. Consequently, those and other components in a capacitor limit the extent to which its physical dimensions may be reduced.
Recently developed flat aluminum electrolytic capacitors have overcome some disadvantages inherent in commercial cylindrical capacitors. For example, U.S. Pat. No. 5,131,388 to Pless et. al. discloses a relatively volumetrically efficient flat capacitor having a plurality of planar layers arranged in a stack. Each layer contains an anode layer, a cathode layer and means for separating the anode layers and cathode layers (such as paper). The anode layers and the cathode layers are electrically connected in parallel.
In a recent paper "High Energy Density Capacitors for Implantable Defibrillators" presented at CARTS 96: 16th Capacitor and Resistor Technology Symposium, Mar. 11-15, 1996, several improvements in the design of flat aluminum electrolytic capacitors are described. Described are the use of a solid adhesive electrolyte for strengthening the separator and allowing use of a thinner separator. Also described are a triple anode formed from a non-porous foil disposed between two porous foils. By increasing the number of anode foils per anode layer, the total number of separator and cathode layers in a given stack assembly is reduced, thereby decreasing thickness and volume. Next described are an embedded anode layer tab, where a notch is cut in the anode and a tab of the same thickness as the center anode is placed in the notch. Three anode layers are welded to one another and to the tabs by a cold welding process. See also U.S. Pat. Nos. 5,562,801; 5,153,820; 5,146,391; 5,086,374; 4,942,501; 5,628,801 and 5,584,890 to MacFarlane et al.
In U.S. Pat. No. 5,522,851 to Fayram, manufacturing improvements in flat capacitors relating to the use of internal alignment elements are disclosed. Internal alignment elements are employed as a means for controlling the relative edge spacing of electrode layers and the housing. In the absence of such alignment elements, precision assembly by hand may be required, thereby increasing manufacturing costs. The housing size must also be increased to provide tolerance for alignment errors, resulting in a bulkier device. The '851 patent also describes the use of an electrically conductive housing for grounding some capacitor elements, such as the cathode terminal.
A segment of today's ICD market employs flat capacitors to overcome some of the packaging and volume disadvantages associated with cylindrical photoflash capacitors. Examples of such flat capacitors are described in the '388 patent to Pless et al. for "Implantable Cardiac Defibrillator with Improved Capacitors," and the '851 patent to Fayram for "Capacitor for an Implantable Cardiac Defibrillators" Additionally, flat capacitors are described in a paper entitled "High Energy Density Capacitors for Implantable Defibrillators" by P. Lunsmann and D. MacFarlane presented at the 16th Capacitor and Resistor Technology Symposium.
Anodes and cathodes of aluminum electrolytic capacitors generally have tabs extending beyond their perimeters to facilitate electrical connection in parallel. In U.S. Pat. No 4,663,824 to Kenmochi, tab terminal connections for a wound capacitor are described as being laser welded to feedthrough terminals. Wound capacitors usually contain two or no tabs joined together by crimping or riveting. Termination of larger numbers of anode tabs is described in the '851 patent as being accomplished through laser welding of the free ends of the tabs, followed by welding of the tabs to an inner terminal. In the '851 patent, cathode tabs are connected by ultrasonic welding to a step in the capacitor housing.
In assembling a capacitor, it is necessary that the anode and cathode remain separated electrically to prevent short circuiting. It is also important that a minimum separation between the anode and cathode be maintained to prevent arcing between the anode and cathode, or between the anode and the case. In cylindrical capacitors, such spacing is typically maintained at the electrode edges or peripheries by providing separator overhang at the top and bottom of the anode and cathode winding. In addition, the anode and cathode are aligned precisely and coiled tightly to prevent movement of the anode, cathode and separator during subsequent processing and use. In flat capacitors, anode to cathode alignment is typically maintained through the use of internal alignment posts (as described, for example, in the '851 patent to Pless et al.) screws (see the '851 patent to Pless et al.) or by using an adhesive electrolyte (see the patents to MacFarlane, supra).
Sealing of capacitor housings is typically accomplished in a variety of ways. Aultman et al. in U.S. Pat. No. 4,521,830 describes a typical aluminum electrolytic capacitor construction employed from about 1960 to about 1985. Those typical constructions employed a plastic header with two molded-in threaded aluminum terminals of the type shown in Collins et al. in U.S. Pat. No. 3,789,502, where plastic is molded around the terminals. Zeppieri in U.S. Pat. No. 3,398,333 and Schroeder in U.S. Pat. No. 4,183,600 teach prior art capacitors in which an aluminum serrated shank terminal extends through a thermal plastic header. In both patents the aluminum terminal is resistance-heated to a temperature such that the length of the terminal is collapsed and the center diameter is increased to press the serrations into the melted plastic. Aultman teaches a header design employing a compression-fit set of terminals disposed in a polymer header.
Hutchins et al. in U.S. Pat No. 4,987,519 describe a glass-to-metal seal terminal connection with a tantalum outer ring being laser welded into an aluminum case. Kenmochi in U.S. Pat. No. 4,663,824 describes the use of a resin casing that has been previously formed from epoxy, silicon resin, polyoxybenzylene, polyether etherkeytone, or polyether sulfone, and that has high heat resistance. The terminals perforate the walls by molding them into the casing.
Pless et al. in U.S. Pat. No. 5,131,388 describe the use of a polymer envelope for encasement of the stack and feedthroughs. A silicon adhesive is used to seal the envelope at the seams. The polymer-enveloped flat stack is then disposed within a stainless steel or Titanium case. Aluminum capacitor terminals are described as being crimped or welded to the feedthroughs. Fayram in U.S. Pat. No. 5,522,851 does not specifically address the issue of feedthrough design. An anode post is described as being electrically insulated from the housing.
U.S. Pat. No. 4,041,956 to Purdy et al. for "Pacemakers of Low Weight and Method of Making Such Pacemakers"; U.S. Pat. No. 4,692,147 to Duggan for "Drug Administration Device"; and U.S. Pat. No. 5,456,698 to Byland et al. for "Pacemaker" disclose various means of hermetically sealing housings for implantable medical devices, including laser welding means.
Various types of flat and spirally-wound capacitors are known in the art, some examples of which may be found in the issued U.S. Patents listed in Table 1 below.
TABLE 1 ______________________________________ Prior Art Patents U.S. Pat. No. Title ______________________________________ 3,398,333 Electrical Component End Seal 3,789,502 Fused Cathode Electrolytic Capacitors and Method of Making the Same 4,183,600 Electrolytic Capacitor Cover-Terminal Assembly 4,385,342 Flat Electrolytic Capacitor 4,521,830 Low Leakage Capacitor Header and Manufacturing Method Therefor 4,546,415 Heat Dissipation Aluminum Electrolytic Capacitor 4,663,824 Aluminum Electrolytic Capacitor and a Manufacturing Method Therefor 4,942,501 Solid Electrolyte Capacitors and Methods of Making the Same 4,987,519 Hermetically Sealed Aluminum Electrolytic Capacitor 5,086,374 Aprotic Electrolyte Capacitors and Methods of Making the Same 5,131,388 Implantable Cardiac Defibrillator with Improved Capacitors 5,146,391 Crosslinked Electrolyte Capacitors and Methods of Making the Same 5,153,820 Crosslinked Electrolyte Capacitors and Methods of Making the Same 5,324,910 Welding Method of Aluminum Foil 5,370,663 Implantable Cardio-Stimulator With Flat Capacitor 5,380,341 Solid State Electrochemical Capacitors and Their Preparation 5,545,184 Cardiac Defibrillator with High Energy Storage Antiferroelectric Capacitor 5,522,851 Capacitor for an Implantable Cardiac Defibrillator 5,584,890 Methods of Making Multiple Anode Capacitors 5,628,801 Electrolyte Capacitor and Method of Making the Same 5,660,737 Process for Making a Capacitor Foil with Enhanced Surface Area ______________________________________
As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and Claims set forth below, at least some of the devices and methods disclosed in the patents of Table 1 and elsewhere herein may be modified advantageously in accordance with the teachings of the present invention.